The present invention relates to an ignition apparatus having a current limiting function for an internal combustion engine which serves to limit a primary current flowing in a primary winding of an ignition coil for limiting a secondary current flowing in a secondary winding thereof.
In general, internal combustion engines such as automotive gasoline engines have a plurality of cylinders for which the order of fuel injection, the order of ignition and the like are controlled in an optimal manner by mean of a computerized electronic control unit as called "ECU".
The ignition timing of the cylinders of such an engine is determined by cutting off the current supply to the primary winding of an ignition coil, and the secondary winding voltage developed across the secondary winding of the ignition coil upon cutting-off of the primary current supply requires to have high energy enough to generate a spark between the electrodes of a spark plug which is connected to the secondary winding of the ignition coil. In addition, it is necessary to limit the secondary winding voltage thus generated to a suitable energy level which does not cause dielectric breakdown of electronic or electric components of the ignition apparatus, the breakdown voltages for the components being determined in accordance with rated resistant voltages predetermined for the components. To this end, a maximum value of the primary winding current has to be limited to a prescribed value. However, the magnitude of voltage, which is supplied from a DC power supply such as a storage battery to the ignition coil for proper ignition, varies depending upon the operating condition of the engine, so it is general practice for the ignition apparatus to have a current limiting function for limiting the primary winding current to an appropriate level in accordance with the operating condition of the engine.
FIG. 3 illustrates the circuit arrangement of a typical example of such a type of ignition apparatus with a current limiting function for an internal combustion engine. In this figure, a DC power source 1 in the form of a storage battery, which generates a source voltage V.sub.B, is connected to an ignition coil 2 which has a primary winding 2a and a secondary winding 2b of which the latter is connected to one of electrodes of a spark plug 3, whose the other electrode is connected to ground. A power transistor 4 comprising a pair of transistors coupled to form a Darlington circuit has a common collector connected to the primary winding 2a of the ignition coil 2, and a base connected through resistors 6, 7 to a node between a resistor 5, which is connected to a node between the storage battery 1 and the ignition coil 2, and a collector of a drive transistor 6 which has an emitter connected to ground. The drive transistor 6 is incorporated in an ECU (not shown).
A current limiter, generally designated by reference numeral 30, is connected between the base and emitter of the power transistor 4 and it is constructed as follows. A current sensing resistor 9 is connected between the emitter of the power transistor 4 and ground for sensing a primary voltage V.sub.D corresponding to a primary current I.sub.1 which is generated by the primary winding 2a of the ignition coil 2 and flows through the power transistor 4. One end of the current sensing resistor 9 is connected through a resistor 10 to a negative or inverted input terminal of a differential amplifier 11. The other end of the current sensing resistor 9 is connected to ground, an emitter of a transistor 13, and the inverted input terminal of the differential amplifier 11 through a resistor 12. The differential amplifier 11 has an output terminal connected to a junction P.sub.1 between the resistors 6, 7. The transistor 13 has a collector connected through a resistor 14 and a constant current supply 15 to the junction P.sub.1 between the resistors 6, 7, and a base directly connected to the collector thereof to form a diode connection. The base of the transistor 13 is also coupled to a base of a transistor 16 which has a collector connected through a resistor 17 to the constant current supply 15, an emitter connected through a resistor 18 to ground, and a base connected through a resistor 19 to a positive or non-inverted input terminal of the differential amplifier 11. The collector of the transistor 16 is also coupled to a base of a transistor 21 which has a collector connected to the constant current supply 15 and an emitter connected to ground.
In operation, when the drive transistor 6 incorporated in the unillustrated ECU is turned off for starting the power supply to the ignition coil 2, the source voltage V.sub.B of the storage battery 1 is imposed on the base of the power transistor 4 through the resistor 5, thus turning the transistor 4 on. As a result, a primary current I.sub.1 begins to flow from the storage batter 1 to ground by way of the primary winding 2a of the ignition coil 2, the collector-emitter of the power transistor 4 and the current sensing resistor 9. A voltage across the current sensing resistor 9 is applied to the inverted input terminal of the differential amplifier 11 through the resistors 10, 12.
At the same time, the current limiter 30 starts to control the base current I.sub.B4 to the power transistor 4 so that the sensed voltage V.sub.D across the resistor 11 corresponding to the primary current I.sub.1, which is applied to the inverted input terminal of the differential amplifier 11 through the resistors 10, 12, is made equal to a reference voltage V.sub.R which is generated across the base-emitter of the transistor 13 and imposed on the non-inverted input terminal of the differential amplifier 11 through the transistor 16 and the resistors 19, 20. That is, when the sensed voltage V.sub.D becomes equal to the reference voltage V.sub.R, a part of base current I.sub.B4, which is to be supplied to the base of the power transistor 4, is absorbed as a so-called sink current I.sub.S by the differential amplifier 11. As a result, the magnitude of the base current I.sub.B4 supplied to the base of the power transistor 4 is accordingly reduced. In this manner, the primary current I.sub.1 is controlled or limited to a level corresponding to the predetermined reference voltage V.sub.R. In this connection, the sensed voltage across the current sensing resistor 9 input to the inverted input terminal of the comparator 11 varies with a change in the temperature of the resistor 9 because the resistance r.sub.9 thereof has temperature dependency. Thus, the temperature-dependent change in the resistance r.sub.9 of the current sensing resistor 9 is compensated for by changing the reference voltage V.sub.R so as to offset the change in the resistance r.sub.9. That is, the reference voltage V.sub.R imposed on the non-inverted input terminal of the differential amplifier 11 is expressed by the following formula: ##EQU1## where k is Boltsmann's constant (=1.38.times.10.sup.-23 J/K); T is the absolute temperature of the transistors 13, 16; q is the charge of an electron (=1.6.times.10.sup.-19 coulomb); Ie.sub.13 is the emitter current of the transistor 13; Ie.sub.16 is the emitter current of the transistor 16; Vbe.sub.16 is the base-emitter voltage of the transistor 16; Is the saturation current of the transistor 16 (=5.38.times.10.sup.-16 amperes at an absolute temperature of 300.degree. K.); r.sub.19 is the resistance of the resistor 19; and r.sub.20 is the resistance of the resistor 20. As can be clearly seen from equation (1) above, the temperature-dependent change in the reference voltage V.sub.R can be compensated for by changing the ratio of the emitter current Ie.sub.13 of the transistor 13 to that Ie.sub.16 of the transistor 16 as well as a voltage dividing ratio determined by the resistances r.sub.19, r.sub.20 of the resistors 19, 20 (i.e., r.sub.20 /(r.sub.19 +r.sub.20)).
In this regard, however, the base-emitter voltage Vbe.sub.16 of the transistor 16 has a negative characteristic with respect to the temperature change thereof, i.e., it decreases as the temperature thereof rises. Therefore, as clearly can be seen from equation (1) above, the temperature coefficient of the reference voltage V.sub.R can not be increased over a certain limit C1 which is expressed as follows: EQU C1=(k/q){log(Ie.sub.13 /Ie.sub.16)
Thus, if the current limiter 30 comprises a hybrid IC with the current sensing resistor 9 being formed of a material such as aluminum, copper and the like having a relatively large temperature coefficient (i.e., greater than the above limit C1), it becomes difficult to make the temperature coefficient of the reference voltage V.sub.R match that of the current sensing resistor 9. In other words, in this case, there inevitably arises mismatching between the temperature coefficients of the voltages applied to the inverted and non-inverted input terminals of the differential amplifier 11, thus giving rise to temperature dependency of the limit value of a primary winding current as limited by the current limiter 30. That is, the current limiting value of the current limiter 30 drifts in accordance with variations in the temperature thereof, and hence it has a temperature depending characteristic which is undesirable. For this reason, it has hitherto been necessary to form the current sensing resistor 9 from materials having a low temperature coefficient such as a precious metal or alloy like a silver-palladium (Ag-Pd) alloy, silver, etc., which, however, are very expensive.
In addition, when considering stability and accuracy in operation of the current limiter 30 of FIG. 3, it is very important to stabilize the voltage applied to the junction P.sub.1 by the storage battery 1. To this end, in order to compensate for or correct the temperature dependency of the base-emitter voltage Vbe.sub.4 of the power transistor 4, it is necessary to substantially increase the resistance of the current sensing resistor 9 and/or employ the resistor 6 connected to the base of the power transistor 4.